The present invention relates generally to communication systems, and more particularly, to a method and apparatus that is employed in an echo canceller to detect and protect against random transitions from linear to non-linear echo paths.
Communication systems generally have opposing ends between which signals are transmitted. Two-wire connections, in particular, require mixing of a duplex telephone signal (i.e., transmit and receive) for communication between wire line subscribers through a central telephone office. In a telephone network, four-wire and two-wire segments are joined at opposite ends of the telephone network by hybrid circuits, which are often referred to as 4:2 hybrids. Impedance mismatch in a hybrid circuit causes a four-wire receive path signal to be reflected onto the four-wire send path. If there is enough delay in the network, this reflected signal presents itself as echo to the speaker who originated speech at a far-end of the network. Short delays experienced between local communicators (on the order of 1 to 20 milliseconds) typically do not present an impediment to the efficient exchange of spoken words. Longer delays, however, may result in syllables and even entire words being repeated as an echo and may render the communication channel unusable. Echo may also be acoustic in nature as a result of microphone and speaker coupling, such as in mobile handsets and hands-free units, for example. Acoustic echo greatly undermines voice quality if the mobile hands-free unit fails to compensate for the acoustic echo.
A solution to the echo problem has been to provide what are referred to as xe2x80x9cecho cancellersxe2x80x9d to prevent delayed or reflected far-end signals from interfering with a near-end listener by attempting to cancel any extraneous signals present on a channel. Specifically, echo cancellers cancel echo by producing synthetic echo for subsequent subtraction from the received echo signal (near-end talker). The echo canceller utilizes a synthetic echo generating system, also known as an adaptive filter, that processes an incoming voice signal, referred to as a reference speech signal x(n), to estimate the echo that will be generated by those elements of the communication system that introduce echo. Generally, as long as the echo canceller is converged, the signal output from the echo canceller is relatively echo free.
In conventional echo cancellers, the adaptive filter utilizes algorithms to estimate the echo based on the assumption that the echo generating system has a linear dynamic characteristic. However, it has been observed by the applicants that an already converged echo canceller may experience divergence due to significant departure of the echo generating system from a linear dynamic to a non-linear dynamic. This departure may be transient or random in nature, wherein the echo generating system reverts back from a non-linear state to a linear state and vice versa. Consequently, the occurrence of this behavior leads to unsatisfactory performance for an echo canceller with a significant amount of uncancelled, annoying echo leaving the echo canceller.